The invention concerns a sleeve made of aluminum or an aluminum alloy for protecting the iron pins of an anode block used as a carbon block or anode in the electrolytic production of aluminum.
The baked carbon anodes used in the electrolytic production of aluminum from a molten, fluoride-containing electrolyte are usually provided with anode conductor bars with two or four pins or plugs made of iron. The iron pins are set in the upper part of the carbon anode and embedded securely there by ramming with a granular or pasty mass of carbon, or by pouring a molten anode carbon paste around them. The cross section of the carbon anode is for example about 500.times.1000 mm to 500.times.1500 mm and the height 400 mm to 550 mm. The carbon anode is connected by means of the iron pins to the anode conductor bars and the latter connected to the electrical supply by way of clamps.
After the carbon anode is mounted in the pot, its head heats up to about 180.degree. C. After six days the temperature there rises on the average to about 400.degree.-500.degree. C. Because of combustion of the anode by the oxygen released in the reduction process, the anode must be lowered by about 15 to 20 mm per day. Finally it lies so low that the iron pins also lie below the surface of the molten electrolyte. The ends of the pins are protected from this mass by the carbon mass surrounding them, but the part projecting out of the anode is not. In order that this does not scale away and dissolve partially in the electrolyte, the part of the anode pins projecting out of the carbon anode must also be protected. This can for example be achieved using a mass of artificially produced carbon (coke/pitch mixture), in that this material is poured into a sleeve around the iron in a recess in the anode. A kind of sleeve made of aluminum sheet--also called a collar--is very suitable for this purpose, as the aluminum does not contaminate the bath when it melts off, and is recovered in the cathodic aluminum layer in the cell.
A sleeve, about 250.times.100 mm in plan view and 100 to 200 mm high, can be used for example for an iron anode pin of 200.times.50 mm. The sheet is, e.g., about 1 mm thick. Usefully hard rolled pure aluminum sheet--the purity and surface condition of which is of almost no importance--is used for the production of the sleeve. Scrap sheet can therefore also be used. Of course alloys of aluminum can also be considered for this purpose, provided they do not contain such large quantities of alloying elements that the aluminum produced in the pot is unduly contaminated.
The recesses in the head of the carbon anode correspond in number and dimensions to those of the sleeves which fit neatly into these rectangular or square recesses.
FIG. 1 shows an aluminum sleeve representing the state of the art and is for an iron pin which is square in cross section. The sleeve is made out of a strip of sheet both ends of which are joined by a fold. This sleeve is shown in horizontal cross section as it comes out after automatic production. In the example shown it is approximately square in cross section. The end part 11 of the wall length 10 is bent inwards and the end part 12 of the wall length 13 is bent outwards. The bent ends 11 and 12 must be engaged on one another and knocked or pressed flat, so that a folded joint is obtained and that the sleeve does not open on being filled with the carbon mass. The horizontal cross section is then almost completely square in shape, and the sleeve can be set in the square recess in the head of the anode.
This and similar known types of sleeve have the disadvantage that, on inserting them into the recess, they have to be adjusted, so that they are at the same distance from the iron pin all round. There is also the disadvantage that they move easily out of position while being filled with the pasty carbon mass; inhomogeneities which impair the mechanical properties of the anode can result from this. The object of the invention is therefore to develop a sleeve, which is made of aluminum or an aluminum alloy, and which protects the iron pins of an anode connecting rod set in the carbon blocks used as anodes in the electrolytic production of aluminum, whereby the said sleeve does not exhibit the above mentioned disadvantages, but can still be manufactured in a simple and economic manner.